Life history and contemporary evolution: Implications for managing a hybrid salamander invasion

Predicting adaptive responses of populations is a primary goal of evolutionary ecology and conservation. While evolution on contemporary time scales is now apparent, the underlying mechanisms and whether change is adaptive or neutral often remain unclear. Typically, habitat modification, introduction of nonnative taxa, hybridization, or a combination of these factors is invoked to explain rapid evolution. I focus on an amphibian system of conservation concern to explore the interplay between ecology and evolution in shaping population genetic structure, including how habitat and physiology interact with life history to promote invasion success. I begin by using simulation to demonstrate that the mode and tempo of evolution is highly subject to interactions between genome structure and life history strategy. I show that incorporating high reproductive outputs characteristic of amphibians can increase the probability that selection and recombination result in highly fit multilocus genotypes, but complex life cycles characteristic of pond-breeding taxa can simultaneously increase the probability that low-fitness combinations drift rapidly to fixation, highlighting the possibility that rapid "adaptation" in small populations might sometimes be explained by drift. I then explore environment-dependent selection in a hybrid zone involving obligately-metamorphic California Tiger Salamanders (Ambystoma californiense) and nonnative Barred Tiger Salamanders (A. mavortium) having a more plastic facultatively-paedomorphic life history. I consider measures of physiological performance that are expected to affect amphibian population dynamics, evaluating whether metabolic and water loss rates are consistent with hybrid advantage or disadvantage (and transgressive segregation) under conditions experienced during key life history events. Alternating patterns of hybrid advantage linked to high energy demand and plastic life history result in bounded hybrid superiority and, as yet, a clear limit to the geographic spread of hybrid genotypes. Next, I show that recent abrupt declines in hybrid population success are likely associated with high energy demand alongside drought-induced reductions in resources, where many [facultatively paedomorphic] hybrid populations experienced pond-drying for the first time since their initial invasion. In contrast, obligately-metamorphic natives that evolved in a drought-prone landscape did not experience apparent declines. These results underscore integrating scientific disciplines as requisite to effective conservation science and invasion management.